MIAMI-DADE COUNTY SEPTIC RETURN FLOWS: PATHLINES, ENDPOINTS, AND FLOWS BASED ON THE URBAN MIAMI-DADE GROUNDWATER MODEL

Miami-Dade County (MDC) has over 105,000 septic systems, some of which are at risk of compromise due to sea level and associated water table rise. MDC is surrounded by protected water bodies with environmentally sensitive species and is underlain by highly transmissive karstic limestone. Septic system leach fields should be adequately above the water table to function properly. If not, the systems may be less effective at attenuating and breaking down wastewater constituents, including bacteria, viruses, nutrients, pharmaceuticals, and personal care products. Failed systems could be contaminant sources that pose risks to aquifers and surface water bodies. They could also contribute to growth of algal blooms, contribute to low dissolved oxygen, and present other environmental and public health issues for nearby surface water bodies and Biscayne Bay.

This research leverages MDC's county-scale surface-groundwater model using pathline analysis to estimate the transport and discharge fate of septic system flows under the complex time history of groundwater flow response to storms and other environmental factors. The model was published by the USGS in 2016 and covers an area of 4,772 km² in Southeast Florida. The model domain has a uniform grid with dimensions of 500 by 500 meters, 3-layers, 189 rows, 101 columns, and 10,958 daily stress periods (30 years).

Outputs from the model were used to create a 30-year (1996-2026) simulation of particle motion from septic discharge locations; MODFLOW-2005, MODPATH, ModelMuse, Python, ArcGIS, and SGeMS were used for the investigation. The MODPATH package was used to release and track particles. Initial particle locations were obtained from the original model’s septic return flow input file by placing one particle in model cells with non-zero septic return flow.

Spatial-temporal pathlines with termination points were generated, allowing us to map flow paths and the spatial distribution of the septic flow discharge. Most septic return flows were discharged to surface water (52,830 m³/d), well fields (14,066 m³/d), and Biscayne Bay (5,696 m³/d). Results allow us to identify “hotspots” to guide water quality sampling efforts and to provide recommendations for areas that will benefit most from septic-to-sewer conversion, reducing nutrient loading on water bodies in the County.